_BACTERIAL GROWTH

_Growth
of Bacteria is the orderly increase of all the chemical
constituents of the bacteria. Multiplication is the consequence
of growth. Death of bacteria is the irreversible loss of ability to
reproduce.

Bacteria
are composed of proteins, carbohydrates, lipids, water and trace elements.

Growth
Factors: A growth factor is an organic compound which
a cell must contain in order to grow but which it is unable to synthesize.
These substances are essential for the organism and are to be
supplied as nutrients. Thiamine, nicotinic acid, folic acid and
para-aminobenzoic acid are examples of growth factors.

Essential Metabolites:
These metabolites are essential for growth
of bacterium. These must be synthesized by the bacterium, or be
provided in the medium. Mg, Fe and Mn are essential trace elements.

Autotrophs live
only on inorganic substances, i.e. do not require organic
nutrients for growth. They are not of medical importance.

2.
pH of the medium. Most
pathogenic bacteria grow best in pH 7.2-7.4. Vibno cholerae can grow in pH
8.2-9.0.

3.
Gaseous Requirement(a) Role of Oxygen. Bacteria may be
classified into four groups on oxygen requirement :(i)
Aerobes. They
cannot grow without oxygen, e.g. Mycobacterium tuberculosis.(ii)
Facultative anaerobes. These
grow under both aerobic and anaerobic conditions. Most bacteria
are facultative anaerobes, e.g. Enterobacteriaceae.(iii)Anaerobes.
They only grow in absence of free oxygen, e.g. Clostridium,
Bacteroides.(iv) Microaerophils
grow best in oxygen less than that present in the air,
e.g. Campylobacter.

Anaerobic bacteria do not grow in the presence of oxygen. They do not
use oxygen for growth
and metabolism but obtain their energy from fermentation reactions.
Anaerobic bacteria are killed by oxygen or toxic
oxygen radicals. Multiple mechanisms play
role for oxygen toxicity : (1) They do not have cytochrome systems for
oxygen metabolism, (2) They may have low levels of superoxide dismutase,
and (3)
They may or may not have catalase.

4.
Temperature.
Most bacteria are mesophilic.
Mesophilic bacteria grow best at 30-37°C. Optimum temperature for
growth of common pathogenic bacteria
is 37°C. Bacteria of a species will not grow but may remain alive at a
maximum and a minimum temperature.

5.
Ionic strength and osmotic pressure.

6.
Light. Optimum
condition for growth is darkness.

B.Sources of Metabolic EnergyMainly three mechanisms generate metabolic energy. These
are fermentation, respiration and photosynthesis. An organism to grow, at least one of these mechanisms must be used.

REPRODUCTIONBacteria
reproduce by binary fission. Multiplication
takes place in
geometric progression. The nucleus (chromosome) undergoes duplication
prior to cell division. When the cell grows about twice its size, the
nuclear material divides,
and a transverse septum originates
from plasma membrane and cell wall and divides the cell into
two parts. The two daughter cells receive an
identical set of chromosomes. The daughter cells separate and may be
arranged singly, in pairs, clumps, or chains.

GROWTH CURVEThe
growth curve indicates multiplication and death of bacteria. When a bacterium
is inoculated in a medium, it passes
through four growth phases which will be evident in a growth curve drawn
by plotting the logarithm of the number of bacteria
against time. Number of bacteria in the culture at different periods may be : (1) Total count. It
includes both living and dead bacteria,
or (2) Viable count. It includes only the living bacteria.
Microbial concentration can be measured in terms
of cell concentration, i.e. the number of viable cells per unit volume of culture, or of biomass
concentration, i.e. dry weight of cells per unit volume of culture.

Bacterial count is of use in
the examination of water and milk.

_Growth Phases

1.
Lag Phase. In
this phase there is increase in cell size but not
multiplication. Time is required for adaptation (synthesis of new enzymes) to new
environment. During this phase vigorous metabolic activity occurs but cells do
not divide. Enzymes and intermediates are
formed and accumulate until they are present in concentration that permits growth to start. Antibiotics have little
effect at this stage.

2.
Exponential Phase or Logarithmic (Log) Phase. The cells multiply at the maximum rate in this exponential
phase, i.e. there is linear
relationship between time and logarithm of the number of cells. Mass increases in an exponential manner. This continues until one of two things happens:
either one or more nutrients in
the medium become exhausted, or toxic metabolic
products, accumulate and inhibit growth. Nutrient oxygen becomes limited for aerobic organisms. In
exponential phase, the biomass
increases exponentially with respect to time, i.e. the biomass doubles with each doubling time. The average time
required for the population, or the biomass, to double is known as the generation time or doubling time. Linear plots of exponential growth can be produced
by plotting the logarithm of biomass concentration as a function of
time. Importance : Antibiotics act better at this phase.

3.
Maximal Stationary Phase. Due
to exhaustion of nutrients or accumulation of toxic products
death of bacteria starts and the growth cease completely. The
count remains stationary due to balance between multiplication and death rate. Importance:
Production of exotoxins, antibiotics,
metachromatic granules, and spore formation takes place in this phase.

4.
Decline phase or death phase. In
this phase there is progressive death of cells. However, some living bacteria
use the breakdown products of dead bacteria as nutrient and remain as persister.